The present invention is directed to compressive receivers. It is particularly concerned with increasing the frequency resolution of such receivers.
A compressive receiver can be thought of as a spectrum analyzer or Fourier-transform device. The input to the compressive receiver is a time-dependent signal, and the output of the receiver is a waveform whose value at a given time indicates the presence of spectral components in the incoming signal at a frequency corresponding to the given time. That is, an input sine wave of a given frequency will result in an output pulse at a particular point in time as the output of the compressive receiver, while a sine wave of a different frequency will cause a pulse in the output at a different point in time.
The typical compressive receiver consists of a dispersive delay line fed by a linearly swept frequency translator, such as a mixer and a linearly swept local oscillator. The dispersive delay line is chosen to have a substantially linear relationship of delay to frequency; that is, the difference between the delays experienced by two simultaneous frequency components having a given frequency difference is proportional to the frequency difference.
The local oscillator is swept in frequency in a sawtooth manner at such a rate that mixer outputs caused by all signals of a given frequency occurring at the input port of the mixer during a single sweep arrive at the output port of the delay line at the same time. The time of arrival of a pulse at the delay-line output port is thus an indication of the frequency of the signal that gave rise to it.
A two-dimensional compressive receiver employs the same principle, but it uses a two-dimensional delay line having a set of input ports distributed along one edge and a set of output ports distributed along the opposite edge. In a typical application, the input ports are driven by elements of an antenna array. Each antenna element provides essentially a time-shifted version of the signals from the other antenna elements. Accordingly, interference patterns are set up within the delay line. The geometry of the two-dimensional delay line is such that, if there is a linear relationship between input-port position and the time delay of a signal arriving at the input port, the signals within the delay line caused by a particular frequency component will all constructively interfere at a particular point along the opposite edge of the delay line, the position of this point depending on the time difference between the various input signals. For instance, if signals of a given frequency all arrive at the same time, constructive interference of all those signals might occur at the center output port, while the point of constructive interference might be at one of the left output ports if the signals at the left input ports are delayed more than those at the right input ports. If the input ports are driven by elements of a linear antenna array, the position of the output port having the greatest signal is thus an indication of the direction of the source of the signal.
Regardless of whether the delay device is of one dimension or more dimensions, the frequency resolution of the device is equal to the reciprocal of the difference, T, between the delays of the lowest and highest frequencies that it can process in a single sweep of the local oscillator. That is, a single-frequency signal at the mixer input port will cause a delay-line output pulse whose duration is long enough to prevent it from being distinguished from a pulse caused by a single-frequency signal of a different frequency unless the difference between their frequencies is greater than 1T. In the past, the only way to increase frequency resolution was to increase T which means to increase the delay-line length.
It is an object of the invention to increase the frequency discrimination in a compressive receiver without increasing delay-line length.
The foregoing and related objects are achieved in a compressive-receiver system in which an input signal of a given input bandwidth is time-compressed before it is applied to the dispersive delay line. That is, the input signal is recorded for a given time interval and then played back at a faster speed so that the playback lasts for a shorter time interval. This applies the information to the delay line in a wider range of frequencies. Since the spread in frequency of the input to the delay line is increased, the spread in delay time for the input bandwidth is also increased. This increases the frequency resolution without lengthening the delay line.